Please see How does Duplicati Resume After Interruption? for a comment from the main author on the design. Whether or not everything always works out well is not known to me, and might need some more investigation.
Making immutable mount points with chattr is something you might try if you think such unmounts may happen.
Database management covers repairing or recreating the local database. For a big backup, recreate can get slow, and this would be one reason why breaking a large backup into several smaller ones is sometimes good.
Docs » Manual » Disaster Recovery covers some of the things that can be tried if the normal recoveries fail…